Crittervision: See like a bee

When a bee flies into your garden, it doesn't see what you and I see. Flowers leap out from much darker-looking leafy backgrounds, and they have ultraviolet-reflecting landing strips that show the way to the nectar. Some spiders might even have evolved to exploit these displays, spinning UV patterns into their webs that could work to fool a bee into thinking that it was making a beeline for a tasty treat.

If the bee manages to resist the spider's trap, she finds her way back home by checking the pattern of polarised light in the sky. All this is seen through the pixellated window of mosaic vision, with each unit of the insect's compound eye providing one of the 5000 dots that make up an image.

It's a world of vision that it is difficult to imagine, but we might get some clues from people with aphakia: a condition in which the lens of the eye - which normally absorbs UV light before it can reach the retina - has been removed in surgery or lost in an accident. Bill Stark, an insect-vision researcher at Saint Louis University in Missouri, lost the lens in his left eye after an accident when he was 10 years old. He says he can see UV light as a kind of "whitish blue", which he would see washing the scenery at a funfair, for example. Because the sight in his left eye is not great, however, he cannot see the subtle patterns in flowers that bees do.

Mind you, even if Stark's vision was corrected his experience of UV could not match that of a bee, says Lars Chittka, a sensory and behavioural ecologist at Queen Mary, University of London. "Bees have a specific UV receptor - humans don't," he explains. "These people see UV with their blue receptors, because the sensitivity of our blue receptors extends weakly into the ultraviolet. But humans can't perceive UV as a separate colour."

The complexity of the bee's colour system is nevertheless comparable to human vision, since, like humans, they only have three colour receptors - for UV, blue and green, compared with the human set-up of blue, green and red. This means that false-colour photographs, in which red has been filtered out and UV has been added in a colour visible to human eyes, gives us a close approximation of the patterns a bee sees.

Besides their UV vision, bees can also detect the polarisation of light. "Just like you see red from blue they see one polarity from another," says Stark. Air molecules in the atmosphere scatter photons to create a pattern of polarised light arranged around the sun, for example (see diagram). This helps bees to navigate by the position of the sun even when the sky is cloudy. Since polarised light is measurable using relatively simple detectors, we can again create images of the kind of information they can pick up.

If a bee's-eye view of the world seems alien to our own, it is nothing compared with that of some insects and birds, which have four, five or even six colour receptors, allowing them to perceive colours that it is impossible for us to experience or even imagine. For them, the three-colour world of human vision would be as dull as greyscale.

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